1. Field of the Invention
The present invention relates to a toner kit for developing an electrostatic image or a toner kit for forming a toner image in accordance with a method for forming an image using a toner-jet system in a method for forming an image such as electrophotography or electrostatic printing. In particular, the present invention relates to a toner kit that comprises a toner to be used in a fixation system in which a toner image is fixed on a transfer material such as a print sheet under heat and pressure. Furthermore, the present invention relates to a method for forming an image of electrophotographic type method for forming an image to be used in a copying machine, a printer, a facsimile machine, a digital-proofing device, etc. and an image forming apparatus of electrophotographic type to which the method is applied.
2. Description of the Related Art
Heretofore, various kinds of electrophotographic methods have been known in the art. Generally, those methods include the steps of: uniformly charging the surface of a latent image bearing member made of a photoconductive material by charging such as corona charging or a direct charging with a charging roller or the like; forming an electric latent image on the latent image bearing member by irradiation with optical energies; forming a toner image by developing the electric latent image with a positively charged toner or a negatively charged toner; optionally transferring the toner image to a transfer material such as a sheet of paper; and fixing the toner image on the transfer material under heat, pressure, or the like. Through those steps, a copy of the original is obtained. Then, the residual toner without being transferred to the transfer material in the transfer step is removed from the transfer material by any of the well-known methods, followed by repeating the preceding steps.
In recent years, electrophotographic image forming apparatuses such as printers and copying machines capable of forming images of higher resolutions are on demand. In particular, for electrophotographic color image forming apparatuses, the demand for excellent image qualities are increasing and the applications thereof are becoming widely various as these apparatuses are becoming widely available. In other words, the reproduction of an image copy of the original such as a photograph, a catalogue, or a map in which the image is reliably reproduced with high precision is on demand for the color image forming apparatus. Concurrently, there are other demands of further increasing the color distinction of the image and further extending the color-reproduction range of the image.
For addressing these needs, there is a method in which an electric latent image is formed by adjusting the density of dots with a constant potential at the time of forming the electric latent image in an electrophotographic image forming apparatus which uses, for example, digital image signals. In this method, however, toner particles are hardly placed on each dot with precision, so that the toner particles may lie off the dot. Therefore, a problem is likely to occur in that the gradation of a toner image corresponding to the ratio of dot densities in black and white portions in a digital latent image.
As a method for addressing the needs described above, for example, there is a method that improves the resolution of an image by decreasing the size of dots that form the above electric latent image. In this method, however, it is difficult to reproduce the electric latent image formed from minute dots, resulting in a poor resolution. Therefore, the resulting image tends to have particularly poor gradation in a highlight portion lacks in sharpness. Furthermore, irregularities in an arrangement of dots cause graininess in the image, which leads to decrease in the image quality of the highlight portion.
For solving these problems, as another method for addressing the needs described above, there is proposed a method that forms an image using a pale toner in a highlight portion and a deep toner in a solid portion.
As the method for forming an image for forming an image, the method in which toners having different concentrations are combined together and used in the process of an image formation has been disclosed in JP 05-25038 A, JP 08-171252 A, JP 11-84764 A, JP 2000-231279, JP 2000-305339 A, JP 2000-347476 A, JP 2001-290319 A, etc.
As an image forming apparatus for the above method for forming an image for forming an image, for example, JP 2000-347476 A discloses an image forming apparatus in which a deep toner is combined with a pale toner such that the maximum reflecting density of the pale toner is half the maximum reflecting density of the deep toner or less. In JP 2000-231279 A, there is proposed an image forming apparatus that utilizes a deep toner having an image density of 1.0 or more and a pale toner having an image density of less than 1.0 in combination when the amount of the toner on a transfer material is 0.5 mg/cm2. Furthermore, in JP 2001-290319 A, there is proposed an image forming apparatus that uses a combination of pale and deep toners in which the ratio between the recording density gradient of the deep toner and the recording density gradient of the pale toner is in a range of 0.2 to 0.5. In these documents, however, there is no teach or description about the amount or concentration of a colorant to be added in the toner and there is no teach or description about a favorable formulation of the toner.
According to the studies of the present inventors, it became evident that these image forming apparatuses had a problem of eminently increasing the graininess of an intermediate density area where the deep toner and the pale toner are mixed even though the gradation and the graininess of a low density area composed of only the pale toner are improved. According to the studies of the present inventors, it became evident that the above image forming apparatuses had been designed insufficiently with respect to an extension of the range of color reproduction.